This invention relates to an inverter control apparatus, or more in particular to an inverter control apparatus most suitably used for an uninterruptible power system.
Such an inverter that is connected with a DC power supply which rectifies a received AC power to produce DC power and converts the DC power supplied by the DC power supply into stable AC power is well known as disclosed, for example, in U.S. Pat. No. 3,718,853. It is common practice to provide such an inverter with control means for detecting the output voltage of the inverter to control it at a desired value. Further, what is called an uninterruptible power system is also known, as disclosed for example in U.S. Pat. No. 3,714,452, in which, with an additional DC power supply adapted to be actuated to supply DC power to the inverter immediately upon the occurrence of such a fault of the main DC power supply which interrupts the production of the DC output thereof, thus interruption of the output of the inverter may be prevented. Such an additional DC power supply includes a battery and sometimes means for charging the battery. This uninterruptible power system has the disadvantage that the input voltage of the inverter undergoes a change resulting in a variation in the output voltage of the inverter as described in detail later, due to the voltage difference occurring between the two DC power supplies when the two DC power supplies are switched. As means for dampening the variation in the output voltage of the inverter in such a case, a method was suggested in which a thyristor rectifier is used as the main DC power supply and the rectified voltage is regulated in accordance with the output voltage of the additional DC power supply with the battery. The disadvantages of this method are a complicated configuration and resulting high cost of the apparatus.